Dental casting alloy

ABSTRACT

In order to improve the fracture strength of a dental casting alloy and to prevent it from becoming excessively hard, a dental casting alloy is provided substantially comprising: from 25 to 32 wt % of Cr from 8 to 12 wt % of W, 
         from 0.05 to 0.4 wt %, in each case, of one or more elements in Group 4 a  and/or    Group 5 a  of the Periodic Table, production-related impurities, the rest being cobalt.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of International ApplicationNo. PCT/EP2003/012465, filed Nov. 7, 2003, which claims Priority ofGerman Application No. 102 52 776.8, filed on Nov. 7, 2002, which areincorporated herein by reference in their entirety and for all purposes.

BACKGROUND OF THE INVENTION

The invention relates to a dental casting alloy for the production ofdental prosthetic frameworks.

Casting alloys for dental prosthetic frameworks, in particular forso-called prosthetic model casts, have been known since 1935.

Such model casting alloys have, along with chromium and molybdenum, highcarbon contents in order to achieve both the rigidity required in dentaltechnology and a low viscosity of the melt, which facilitates accuratecasting of the model.

Manganese and silicon are used in these alloys in order to improve thefluidity thereof.

The use of an alloy for the production of castings for dentalapplications is disclosed in DE 36 09 184 C2. This alloy contains from4.5 to 5.5 wt % of molybdenum.

An alloy for dental castings is disclosed in DE 198 15 091 C2, whichspecifies a molybdenum content of from 4 to 8 wt %. In addition, afraction of from 0.05 to 1.2 wt % of tantalum, niobium, and/or tungstenis required, the fraction of each individual element tantalum, niobium,or tungsten being less than 0.5 wt %.

In general, special demands are imposed on alloys to be used in dentaltechnology. For example, alloys to be fused with porcelain or ceramicsmaterials must be compatible with commercial dental ceramics in terms ofthermal expansion and contraction. Furthermore, these alloys must form athin surface layer of oxide to ensure the formation of a bond betweenthe metal and the ceramics material. Also, for aesthetic reasons, thecolor of the oxide should not show through the opaque ceramics material.A certain activation capacity and spring hardness are required fordental castings that are not to be fused with porcelain, for example,removable prostheses with clasps. Furthermore, it is particularlyimportant in dental technology for the alloys used to be capable ofbeing processed with the facilities available in the dental laboratory,ie they should be capable of being cast in conventional castingcentrifuges. Therefore, the alloys commonly used in dental technology asmodel casting materials have a much higher carbon content than istolerable under various other standards. Furthermore, dental castingalloys are to be preferred that have a hardness in their cast state thatdoes not differ substantially from the hardness of natural tooth enamel,so that there is no significant abrasive wear of the tooth arising fromcontact between the dental casting alloy and the tooth surface.Furthermore, it is advantageous when the alloy can be produced with alow nickel content so that patients allergic to nickel can also befitted with such prostheses.

BRIEF SUMMARY OF THE INVENTION

It is an object of the present invention to provide, within thelimitations mentioned above, a casting alloy of the type described abovethat exhibits a high elongation at break and a Vickers hardness number(HV 10) that is not excessively high.

This object is achieved by the invention in respect of the dentalcasting alloy defined above, in that this alloy is composedsubstantially of

25-32 wt % of chromium

8-12 wt % of tungsten,

one or more elements selected from elements in Group IVa and/or Va ofthe Periodic Table as a fraction of 0.05-0.4 wt % in each case, the restbeing cobalt.

In addition, production-related impurities are present in amounts ofless than 0.1 wt %, more preferably less than 0.05 wt % in each case.Preferably the total amount of such impurities amounts to not more than0.4 % by weight, more preferably to not more than 0.2 % by weight. Theamount of an alloy element included in preferred dental alloys asdescribed below are of course to be ignored when calculating the totalamount of impurities even if its amount is small, comparable with oreven lower than the amount allowed for an impurity. Also the upper limitfor each of the impurities given above are not applicable to thoseelements.

In a preferred dental casting alloy of the invention, the total contentof elements in Group IVa and/or Group Va of the Periodic Table is ca0.05-0.4 wt %.

It is further preferred that the elements in Groups IVa and Va of thePeriodic Table be selected from the group consisting of titanium,zirconium, niobium, and tantalum.

Furthermore, alloys of the invention are preferred that aresubstantially free from the elements iron, vanadium, nickel, molybdenum,and carbon.

Other preferred alloys contain

-   from 0 to 1 wt % of Mn-   from 0.8 to 1.6 wt % of Si and/or-   from 0.1 to 0.35 wt % of N.

The addition of Mn has a deoxidizing effect on the melt and binds oxygenwith slag formation.

The silicon fraction serves to lower the viscosity of the melt with theresult that very fine details can be reproduced well in the cast.

The nitrogen fraction increases the ductility. If the nitrogen contentis too low, there are obtained a poorer elongation at break and a lowerelastic limit.

Highly corrosion-resistant prosthetic frameworks can be made with thedental casting alloys of the invention; furthermore, these alloysexhibit good working properties, in particular a low hardness number anda lower thermal expansion coefficient, and they are also easilylaser-welded.

The low thermal expansion coefficients simplify fusing of the frameworkwith ceramics material/porcelain.

DETAILED DESCRIPTION OF THE INVENTION

A particular composition of the alloy of the invention is given below(Table 1) by way of example, and it is emphasized that the invention isnot, of course, confined to this particular alloy. The mechanicalproperties of the alloy depicted in Table 1 are summarized in Table 2below.

Table 3 summarizes Comparative Examples 1 through 6 and, by comparingthem with the above example of an alloy of the invention, illustratesthe importance of complying with the recipe specified in order to obtainthe well-balanced, beneficial properties of the dental casting alloy.TABLE 1 Elementary Percent by Ingredients weight (wt %) Co 60 Cr 28 Mo —W 9.8 Mn 0.3 Si 1.5 N 0.2 Nb 0.2

TABLE 2 Mechanical/Physical Properties Elastic Limit R_(p) 0.2 (MPa) 620Tensile Strength R_(m) (MPa) 845 Modulus of Elasticity (GPa) 190Elongation at Break A₅ (%) 10.2 Vickers Hardness Number HV 10 300Thermal Expansion Coefficient (TEC) within the 14.1 range of 20-500° C.(10⁻⁶K⁻¹)

TABLE 3 Comparative Examples #1 #2 #3 #4 #5 #6 Co 70 68 59.5 68 61 65 Cr20 20 20 20 24 28 Mo 4.5 5 5 — 7 4.5 W 5 5 5 10 5 — Mn 0.3 0.3 0.3 0.30.25 0.25 Si 0.1 1.5 — 1.5 1.5 1.6 N 0.1 0.2 0.2 0.2 0.25 0.2 Fe — — 10— — 0.35 other Ce, C Al, La, Ce HV 10 289 297 280 291 340 310 TEC 15.0514.95 15.43 15.4 14 14.7 (10⁻⁶ K⁻¹) R_(P) 0.2 (MPa) 380 480 467 457 700520 R_(m) (MPa) 630 636 698 675 900 730 A₅ (%) 17.5 10.5 18.1 15.7 7 11

The alloys summarized in the table above depicting the comparativeexamples are all suitable for framework material and for fusing it toceramic or porcelain, but they all exhibit one or more of variousdrawbacks when compared with the alloy of the invention.

Although the alloy of Comparative Example 5 has a low thermal expansioncoefficient, it nevertheless has a relatively high hardness number.

The alloy of Comparative Example 6 exhibits a relatively low hardnessnumber of 3 10 HV 10, but it has the disadvantage of having a highthermal expansion coefficient, which could possibly cause difficultiesduring ceramic fusing.

The Comparative Examples 1 through 4 relate to alloys having lowhardness numbers, but they exhibit much higher thermal expansioncoefficients than the alloy of the invention and are therefore difficultto process with ceramic materials.

This also holds true for the molybdenum-free alloy of ComparativeExample 4, which, compared with the alloy of the invention, has a lowerthermal expansion coefficient and surprisingly, in spite of its hightungsten content, exhibits a low hardness number and an astonishinglyhigh elongation at break of more than 10%.

The addition of a Group IVa or Group Va element selected, in particular,from the group consisting of the elements titanium, zirconium, niobium,and tantalum, as proposed herein, improves corrosion resistance andincreases thermal resistance when the alloy is subjected to fusing toceramic materials or porcelain.

Moreover, the element niobium is comparatively biocompatible, as istitanium or tantalum or zirconium, and causes no tissue reactions.

Consequently, the alloy of the invention is highly biocompatible.

The preferred elements in Group IVa and Group Va of the Periodic Table,that is, niobium, titanium, tantalum, and zirconium, also bind theresidual carbon in the alloy and thus inhibit the formation of chromiumcarbides. This also makes it easy to laser-weld the alloy of theinvention.

Moreover, the presence of the elements niobium, titanium, tantalum,and/or zirconium provides for a strong passivation of the alloy of theinvention. As a result, the alloy shows a very high resistance tocorrosion.

Finally, it is emphasized that the alloy of the invention contains noprecious metal fractions, as is the case in some conventionallow-hardness alloys containing up to 2% of gold in order to reduce thehardness of the alloy to a manageable level.

1. A dental casting alloy, for the production of dental prostheticframeworks, substantially comprising: from 25 to 32 wt % of Cr, from 8to 12 wt % of W, from 0.05 to 0.4 wt %, in each case, of one or moreelements in Group 4a and/or Group 5a of the Periodic Table,production-related impurities, the rest being cobalt.
 2. The alloy asdefined in claim 1, having a total content of Group 4a and/or Group 5aelements of from about 0.05 to about 0.4 wt %.
 3. The alloy as definedin claim 1, wherein the Group 4a and/or Group 5a elements are selectedfrom the group consisting of titanium, zirconium, niobium, and tantalum.4. The alloy as defined in claim 3, wherein the alloy is substantiallyfree from the elements iron, vanadium, nickel, molybdenum, and carbon.5. The alloy as defined in claim 1, wherein the alloy additionallycontains a fraction of Mn in the range of from 0 to 1.0 wt %.
 6. Thealloy as defined in claim 1, wherein the alloy additionally contains afraction of Si in the range of from 0.8 to 1.6 wt %.
 7. The alloy asdefined in claim 1, wherein the alloy additionally contains a fractionof N in the range of from 0.1 to 0.35 wt %.